This project has as its overall goals the identification of the molecular pathogenesis of lymphoproliferative diseases and the development of objective criteria for lymphoma diagnosis and prognosis, using the specific molecular and immunohistochemical characteristics identified in different lymphoma subtypes. We have expanded our analysis of gene expression profiles in lymphoma through collaboration with the laboratory of Louis B. Staudt, NCI. This study uses gene expression array technology to identify clinically and biologically significant gene expression patterns in lymphomas, new clinicopathological entities, and to accelerate the discovery of molecular markers for routine lymphoma diagnosis. In the past year we have characterized the earliest events in the development and evolution of follicular lymphoma. From 1992 to 2000, we identified 23 lymph node biopsies containing focal germinal centers (GCs) strongly bcl-2 protein +, while most of the remaining lymph node showed bcl-2 - follicular hyperplasia. We propose the designation in-situ follicular lymphoma (FL) for this phenomenon. To investigate the clonal nature of the bcl-2 + follicles, we performed laser capture microdissection of bcl-2+ & bcl-2- follicles from the same lymph node in 5 cases, and analyzed them in parallel by PCR amplification of immunoglobulin heavy chain (IgH) genes. In 4 /5 cases the bcl-2 + follicles contained monoclonal IgH gene rearrangements, while the bcl-2 - GCs exhibited a polyclonal ladder. BCL-2 gene rearrangement was detected in three cases by PCR. 18 patients had clinical follow-up; 6 patients developed overt FL at an adjacent or distant site within one year, and 2 patients at 13 and 72 months respectively. 10 patients have not yet shown other evidence of FL. These results suggest that at least close to half of these cases (8/18; 44%) represent homing to and early colonization of reactive GCs by FL. Other cases might represent FL at the earliest stage of development, or a preneoplastic event, requiring a second hit for neoplastic transformation. In 2 additional cases in-situ FL was identified in lymph nodes containing another low grade B-cell lymphoma, chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL) and lymphoplasmacytic lymphoma respectively. Findings from this study not only have significant clinical implications, but also may provide insight into the biology and pathogenesis of early FL.